Suppose I own $1 million worth of Bitcoin I mined back in the day. If I sell the day before I die I’ll owe $200,000 in long term capital gains taxes. But if I die, leave those coins to you, and you sell them the day you inherit them for the same $1 million, you owe no capital gains taxes.
Edit: if this is surprising, it should be. The step up basis is insane.